A virtual machine (“VM”) is a software simulation of a computer that runs on a host computer, but behaves as if it were a separate, independent, computer. Multiple VMs can be run on a single host computer. OS virtualization is achieved by inserting a layer of software, referred to as the virtualization layer or “hypervisor,” between the OS and the underlying server hardware. The hypervisor is responsible for enabling multiple OS images and their respective applications to share the resources of a single computer, such that each OS image functions as if it has at its disposal all of the resources of the computer, while the hypervisor transparently ensures that the system's resources are properly shared among the different OS images and their applications.
A VM generally comprises a configuration file and one or more disk or image files. The configuration file contains information needed by the hypervisor to run the VM, as well as information identifying the virtual resources required by the VM. The image files comprise the actual operating system and application files.
In certain applications, it is common to maintain multiple VM images of which only a few are in an online state (or running state), while the remaining are in an offline state. For example, Microsoft allows keeping hundreds of images of software irrespective of the number of licenses. However, it allows, at any time, only as many active instances of the software as the number of licenses. The VM images often need maintenance, such as installation, uninstallation, and updation of software. For instance, there may be need for installation, uninstallation, and/or updation of anti-virus definitions in offline VM images. In such scenarios, a significant amount of time and effort is required to bring each offline VM image online and then update the software.